Abstract Many environmental and genetic factors underlie the degeneration of nigral DA neurons in Parkinson's disease (PD). Exposure to pesticides such as rotenone and organochlorine herbicides such as 2,4-D, a major component of Agent Orange used in Vietnam War, has been linked to higher incidence of PD. Nigral DA neurons have massive axon arborization and are particularly vulnerable to microtubule-depolymerizing agents. Previous studies show that many environmental PD toxins impact on microtubules. Our previous studies in vitro and in rat neuronal cultures have shown that parkin binds to and stabilizes microtubules. In our preliminary studies using iPSC-derived midbrain DA neurons from normal subjects and PD patients with parkin mutations, we found that parkin mutations markedly reduced the total length and complexity of neuronal processes by destabilizing microtubules. Furthermore, environmental PD toxins, such as 2,4-D, had much higher toxicity on midbrain DA neurons from PD patients with parkin mutations than those from normal controls. To demonstrate that these phenotypes are indeed caused by parkin, we will generate isogenic iPSC by repairing parkin mutations in PD patient iPSCs and by introducing PD-causing parkin mutations in control iPSCs. These lines of isogenic iPSCs will be used to examine the impact of PD environmental toxins on human midbrain DA neurons in vitro and in rat brains. The proposal aims to study how parkin and environmental PD toxins impact on a common molecular target ? microtubules ? to affect the survival of midbrain dopaminergic neurons derived from isogenic pairs of iPSCs. The unique morphology of a single human nigral DA neuron, with its estimated 4.6 meter long axon arborization, renders the cell particularly vulnerable to genetic (e.g. parkin mutations) or environmental factors (e.g. rotenone, 2,4-D) that destabilize microtubules. Mechanistic insights gained from the study on the protective effects of parkin would be useful for the identification of disease-modifying therapies for PD.